The present invention relates to a method for transmitting control information in a communication system comprising at least one base station subsystem and a wireless terminal, in which method a set of alternative values are defined for said control information, information is transmitted in packet form between the base station subsystem and the wireless terminal, the packets to be transmitted on a communication channel are transformed into bursts, and at least one burst formed of a packet is supplemented with at least one item of control data, wherein at the receiving state, the control data received in the burst is examined. The invention also relates to a communication system as set forth in the preamble of claim 8, a wireless terminal as set forth in the preamble of claim 11, as well as a base station subsystem as set forth in the preamble of claim 12.
Appended FIG. 1 shows a communication system implementing packet-format data transmission. This example system is a so-called GPRS packet network (General Packet Radio Service). The system consists of mobile stations MS which communicate with base transceiver stations BTS by means of an air interface (radio interface) Um. The base transceiver stations are controlled by base station controllers BSC which communicate with a mobile switching centre MSC. The base station controller BSC and the base transceiver stations BTS coupled therewith are also jointly called a base station subsystem BSS. The connection interface between the mobile switching centre MSC and the base station subsystem BSS is called an A interface. Correspondingly, the interface between the base station controller BSC and the base transceiver station BTS is called an Abis interface. The mobile switching centre MSC takes care of e.g. controlling incoming and outgoing calls like a centre for a public switched telephone network (PSTN) (not shown). Furthermore, the mobile switching centre MSC takes care of performing the operations required in mobile communication, such as controlling the location of the mobile station e.g. by means of a home location register HLR and a visitor location register VLR.
In digital mobile communication systems, the radio connection has typically been a so-called circuit-switched connection, which means that the resources allocated for each mobile station for a call are reserved for the whole time of the connection solely for this mobile station. The general packet radio service GPRS is a new service designed for digital mobile communication systems. This general packet radio service GPRS is designed particularly for GSM systems. A corresponding packet radio service in the North American D-AMPS system is called CDPD.
The general packet radio service GPRS is a new service under development in the GSM mobile communication system. The appended FIG. 1 shows connections of a telecommunication network in a packet-switched GPRS service. The main element for GPRS services in the network infrastructure is a GPRS support node, so-called GSN. It is a mobility router which implements the coupling and cooperation between different data networks, e.g. to a public switched packet data network PSPDN via connection Gi or to a GPRS network of another operator via a connection Gp, mobility management with the GPRS registers via a connection Gr, and the transmission of data packets to wireless communication devices MS irrespective or their location. Physically, the GPRS support node GSN can be integrated with the mobile switching centre MSC, or it can be a separate network element based on the architecture of the data network routers. The user data is passed via a connection Gb directly between the support node GSN and the base station subsystem BSS consisting of base transceiver stations BTS and the base station controller BSC, but between the support node GSN and the mobile switching centre MSC there is a signalling connection Gs. In FIG. 1, continuous lines between the blocks illustrate data traffic (i.e. the transfer of speech and/or data in digital format) and broken lines illustrate signalling. Physically, the data can be passed transparently via the mobile switching centre MSC. The reference Gn represents a connection between different support nodes of the same operator. The support nodes are normally divided into gateway support nodes GGSN (Gateway GSN) and serving support nodes SGSN (Serving GSN), as shown in FIG. 1.
Consequently, the GPRS service makes it possible to transmit information in packet form between a wireless communication device and an external data network, wherein certain parts of the mobile communication network constitute an access network.
The function of the wireless communication device MS and the support node SGSN can be divided into several layers, each of which have a different function, as shown in FIG. 2. Transmission of data, such as control signalling and the transmission of data transmitted and received by a user, between the wireless communication device MS and the support node SGSN takes place preferably in the form of data frames. The data frame of each layer consists of a header and a data field.
Information contained in a data field can be e.g. information entered by the user of the wireless communication device or signalling data. The following is a description on the functions of the layers in the GPRS system.
In the link layer, the lowermost layer is the MAC layer (Media Access Control) which takes care of the use of the radio channel in communication between the wireless communication device MS and the base station subsystem BSS, such as channel allocation in the transmission and reception of packets.
In the lowermost layer, data transmission between the base station subsystem BSS and the support node SGSN takes place in the L2 layer (link layer) applying a link layer protocol, such as the LAPD protocol, frame relay protocol, or the like, The L2 layer can also contain quality or routing data according to the GPRS specifications. The L2 layer has properties of the physical layer and the link layer according to the OSI model.
Above the MAC layer there is the RLC layer (Radio Link Control) which serves the functions of dividing the data frames of the link layer formed by the LLC layer into packets of the radio connection (PDU, Protocol Data Unit) having a determined size to be transmitted on the radio channel, transmitting the packets, and retransmitting them, if necessary. In the GPRS system, the length of the packets is the length of one GSM time slot (ca. 0.577 ms). In the GPRS system, the packet structure of the radio connection to be applied at the time of the invention is presented more closely in the ETSI standard publication TS 101 350 v6.1.0 (1998-10): xe2x80x9cDigital cellular telecommunications system (Phase 2+); General Packet Radio Service (GPRS); Overall description of the GPRS radio interface; Stage 2 (GSM 03.64 version 6.1.0 Release 1997)xe2x80x9d, section 6.5.4 xe2x80x9cRadio Block Structurexe2x80x9d, p. 20.
The LLC layer (Logical Link Control) offers a reliable communication link between the wireless communication device MS and the support node SGSN. For example, the LLC layer supplements the message to be transmitted with error checking data, whereby attempts can be made to correct incorrectly received messages and, if necessary, the message can be retransmitted. Furthermore, data encryption and decryption takes place in the LLC layer.
The SNDCP layer (Sub-Network Dependent Convergence Protocol) is used for performing the protocol changes, compression, segmenting of the information to be transmitted, and the segmenting of messages coming from an upper layer. The SNDCP frame comprises preferably an SNDCP header and a SNDCP data field. The SNDCP header consists of protocol information (Network Service Access Point Identity, NSAPI) and SNDCP control data, such as the compressing, segmenting and encryption definitions. The SNDCP layer is used as a protocol adapter between protocols used in an upper layer (IP/X.25) and the LLC layer (link layer).
The information to be transmitted comes to the SNDCP layer preferably as data packets complying with a protocol (PDP, Packet Data Protocol) from another application, such as messages according to the X.25 protocol or packets according to the Internet protocol (IP). The application can be e.g. a data application of the wireless communication device, a telecopy application, a computer program communicating with the wireless communication device, etc.
The SNDCP frame is transferred to the LLC layer, in which the frame is supplemented with an LLC header. The LLC header comprises e.g. an LLC control part which determines the frame number and the command type (info, acknowledgement, retransmission request, etc). In connection with logging in the GPRS packet network, the wireless communication device transmits a login request message to the support node SGSN. The support node SGSN can, on the basis of the international mobile station identity (IMSI) of the wireless communication device, retrieve information from the home location register HLR corresponding to the wireless communication device in question, wherein the support node SGSN can, using this information, select a temporary logical link identity (TLLI) for the data transmission connection. If the wireless communication device has previously been allocated a TLLI identity, the wireless communication device transmits this in the request message, wherein the support node SGSN can allocate this identity again for the wireless communication device, or allocate a new TLLI identity. The support node SGSN transmits the TLLI identity selected by the same to the wireless communication device, for use in the data transmission connection between the wireless communication device and the support node SGSN. This TLLI identity is used in the communication to determine the data transmission connection to which each message belongs. The same TLLI identity must not be used in more than one data transmission connection simultaneously. After the connection has been terminated, the TLLI identity used in the connection can be allocated for a new connection to be set up.
The cells of the packet network are divided into routing areas in such a way that each routing area comprises several cells. Thus, the mobility management functions of the wireless communication device are used to maintain information on the location and connection status of wireless communication devices in the operation range of the packet network. This information is maintained both in the wireless communication device and in the packet network, preferably in the GPRS support node SGSN.
To use GPRS services, the wireless communication device first performs logging in the network (GPRS attach), whereby the wireless communication device reports that it is ready for packet data transmission. The login makes a logical link between the wireless communication device and the support node SGSN, making it possible to transmit short message services (SMS) via the GPRS network, paging services via the support node, and informing about incoming packet data to the wireless communication device. In connection with logging in of the wireless communication device, the support node also performs mobility management (MM) and user identification. To transmit and receive data, the packet data protocol (PDP) is activated, whereby the wireless communication device is allocated a packet data address to be used in the packet data connection, wherein the address of the wireless communication device is known in a gateway support node. Consequently, upon the logging in, a data transmission connection is set up to the wireless communication device, to the support node and to the gateway support node, the connection being allocated a protocol (for example X.25 or IP), a connection address (e.g. X.121 address), quality of service, and a network service access point identifier (NSAPI). The wireless communication device activates the packet data connection by an activate PDP context request, in which the wireless communication device gives the temporary logical link identity (TLLI), the packet data connection type, the address, the required quality of service, the network service access point identifier, and possibly also an access point name (APN).
The GSM system is a time division multiple access (TDMA) system, in which traffic on the radio channel is time divided, taking place in repeated TDMA frames, each consisting of several (eight) time slots. In each time slot, an information packet is transmitted in a radio-frequency burst of a definite duration and consisting of a group of modulated bits. The time slots are primarily used as control channels and traffic channels. The traffic channels are used for the transmission of speech and data, and the control channels are used for signalling between a base transceiver station BTS and wireless communication devices MS1. One packet of a radio connection is transmitted in four bursts.
In comparison with a circuit-switched connection, packet data transmission offers e.g. the advantage that resources are allocated for the connection only according to the need, wherein resources can be deallocated for other connections when there is a connection with no need for data transmission. In packet data transmission, the information to be transmitted, such as speech or data, is divided into packets. After a packet has been transmitted via the air interface Um and it the transmitter has no further packet to be transmitted at the moment, the radio resource can be deallocated for the use of other connections. Such packet data transmission is particularly suitable for the use of data services.
The information to be transmitted in the packet connection is transferred from the application layer to the link layer, in which the information is converted into packets of the link layer. For example in the GPRS system, these packets of the link layer intended for data transmission comprise an MAC header, an RLC header and a data field. For encoding the packets, four channel encoding schemes are defined: CS-1, CS-2, CS-3 and CS-4. Each of the channel encoding schemes has certain encoding parameters which are presented in the appended table 1. For example the first channel encoding scheme CS-1 uses the code ratio 1/2 without puncturing. The data transmission rate at this encoding scheme is 9.05 kbit/s in the GPRS system. In a corresponding manner, using the fourth channel encoding scheme CS-4, it is possible to achieve the greatest data transmission rate at the moment, ca. 21.4 kbit/s. The data transmission rate can be increased even further e.g. by allocating several time slots for the connection. The channel encoding scheme used at the time is indicated with so-called stealing flag bits in connection with the bursts. Thus, the receiving device can use these stealing flags to examine, which decoding method should be selected for the reception of the packet. In the GPRS system, the number of stealing flag bits is 8, and 2 of these bits are transmitted in each burst. These bits are added into the bursts at the transmission stage, as will be described below in this specification.
In the GPRS system, the channel encoding schemes are given by means of the following bit strings:
bit no: 1234 5678
CS-1: 1111 1111
CS-2: 1100 1000
CS-3: 0010 0001
CS-4: 0001 0110
These bits are transmitted in the order from the left to the right in four bursts in such a way that in the first burst the first and second bits from the left are transmitted, in the second burst the third and fourth bits are transmitted, in the third burst the fifth and sixth bits are transmitted, and in the fourth burst the seventh and eighth bits are transmitted. This data transmission giving the channel encoding scheme according to the GPRS system is presented in the ETSI standard publication Draft EN 300 909 v6.2.0 (1999-04): xe2x80x9cDigital cellular telecommunications system (Phase 2+); Channel coding (GSM 05.03 version 6.2.0 Release 1997)xe2x80x9d, section 5.1 xe2x80x9cPacket data traffic channel (PDTCH)xe2x80x9d, pages 26-30.
The channel encoding schemes applied in the GPRS system at the time of the invention are presented more closely in the above-mentioned ETSI standard publications TS 101 350 v6.1.0 (1998-10): xe2x80x9cDigital cellular telecommunications system (Phase 2+); General Packet Radio Service (GPRS); Overall description of the GPRS radio interface; Stage 2 (GSM 03.64 version 6.1.0 Release 1997)xe2x80x9d, section 6.5.5 xe2x80x9cChannel codingxe2x80x9d pages 20-22, and Draft EN 300 909 v6.2.0 (1999-04): xe2x80x9cDigital cellular telecommunications system (Phase 2+); Channel coding (GSM 05.03 version 6.2.0 Release 1997)xe2x80x9d, section 5.1 xe2x80x9cPacket data traffic channel (PDTCH)xe2x80x9d, pages 26-30.
The decoder compares the bit pattern formed by the bits received by the decoder into these four schemes and preferably selects the one in which the bit patterns correspond to each other as closely as possible (Nearest Neighbour). One method for performing the comparison is the method of Euclidean distance. Also cross-correlation or Hamming distance can be used to find out the closest scheme.
In the bursts transmitted from the base stations is also transmitted information on the mobile station which is allowed to transmit in the next bursts allocated for transmission of a packet of the link layer. In the GPRS system, eight different values are used to transmit this information, wherein 8 mobile stations can share the same time slot. Each mobile station is allocated its own uplink state flag (USF). For this reason it is very important that the mobile stations can decode this uplink state flag correctly so that no mobile station transmits in bursts allocated for another mobile station. Said 8 uplink state flags (USF) in the GPRS system are selected in such a way that the bit pattern to be transmitted is the same, irrespective of channel coding. In this way, the uplink state flag to be transmitted with the encoded packet on the radio channel is one of the following, presented in such a way that the least significant bit is on the left:
By using these received bits of the uplink state flag, the receiving mobile station performs a deduction, whether the next burst string on the transmission channel is intended for use by the mobile station or not. Also in this comparison it is possible to use said method of Euclidean distance.
To form the bursts from the packets of the link layer, the following steps are preferably taken at the transmission stage. The packet is supplemented with a block check sequence (BCS), which is used to find out possible transfer errors at the receiving stage. Next, the uplink state flag is added to the packet, and channel encoding is performed with any of said channel encoding schemes. If the first channel encoding scheme CS-1 is used, the uplink state flag is added in the MAC header of the packet. With the second CS-2 and third channel encoding schemes CS-3, the uplink state flag is pre-coded, wherein a 6-bit digit is formed, which becomes a 12-bit digit in the channel encoding. In the case of the fourth channel encoding scheme, a 3-bit digit is converted directly e.g. by means of the above-presented table into a 12-bit digit which is added into the packet. Using the first CS-1, second CS-2 or third channel encoding scheme CS-3, the packet is supplemented with a sufficient number of tail bits (in this example 4 bits), after which the packet is subjected to error correction coding by a convolution code using the bit ratio 1/2. After this, if necessary, some of the encoded bits are eliminated (punctured) before modulation to form a burst of a given length. This puncturing is made in the GPRS system when either the second channel encoding scheme CS-2 or the third channel encoding scheme CS-3 is used.
Radio communication is subject to interference, and the mobility of the wireless terminals may cause variations in the signal strength. Thus, the receiving device is not necessarily capable of receiving these stealing flag bits correctly, which may lead to a misinterpretation. Thus the decoder of the receiving device will use an incorrect decoding method in the reception of the packet. Also the bits of the uplink state flag can be received incorrectly, which may cause that the wireless terminal will transmit at a time allocated for another wireless terminal.
It is an aim of the present invention to achieve a communication system for packet data transmission and a method for finding out the channel encoding data and/or the uplink state flag in a reliable way in this communication system. The invention is based on the idea that in addition to finding out the received channel encoding data and the uplink state flag, the strength of the received signal and the signal/noise ratio are measured at the receiving stage, wherein these measurements are used to evaluate if the reception was sufficiently reliable and if the detected control information value, such as channel encoding data and/or the uplink state flag, can be used. The method according to the present invention is primarily characterized in that the method also comprises the step of examining at least one property of the received signal to find out the reliability of the received control data. The communication system according to the present invention is primarily characterized in that the communication system also comprises means for examining at least one property of the received signal to find out the reliability of the received control data. The wireless terminal according to the present invention is primarily characterized in that the wireless terminal also comprises means for examining at least one property of the received signal to find out the reliability of the received control data. Furthermore, the base station subsystem according to the present invention is characterized in that the base station subsystem also comprises means for examining at least one property of the received signal to find out the reliability of the received control data.
The present invention leads to significant advantages. Using the method of the invention, it is possible to improve the reliability of information received in the receiving device particularly in situations in which radio transmission is subject to interference. Thus, it is possible to reduce the probability of the fact that the receiving device selects an incorrect decoding method. Also the probability that the wireless terminal tries to transmit at a time allocated for another wireless terminal is reduced by applying the method of the invention, particularly in situations in which the signal strength and/or the signal/noise ratio is small.